#3995 fix(apple): read the runner cache toolchain key from Xcode's own files

Summary

Perf Nightly and an Apple smoke run built the runner, then failed writing its cache metadata: xcrun --show-sdk-version hung for 30 s, hung again on retry, and exhausted the 45 s budget. #2423 and #3973 added budgets, retries, and traces, but no budget can size this hosted-runner stall.

• The runner cache key now reads the files those tools print from (version.plist, SDKSettings.json, the SDK's SystemVersion.plist) and never execs. Deleted: the probe budget, retry, TTL memo, selection records, and the runner host's runCmdSync (−2,145 lines).
• A dangling xcode-select link fails as apple_toolchain_unreadable instead of silently reading another Xcode.
• The apple-runner tests default to a fixture Xcode.
• chore(gates): oxlint bans exec-capable imports in the identity module, kept as one module so the platform-apple facades evaluate no more modules than at the merge-base.

Out of scope: the native-build identity (snapshot bridge, fold helper) goes through the Apple tool provider, which may run on another host, so it keeps xcodebuild -version. 37 files. No issue.

Validation

• pnpm check:affected --run --head 805c818902: passed, exactCommit=true.
• Xcode 27.1: file values match xcodebuild/xcrun for all 7 runner SDKs.
• Planted host-Xcode reads and exec imports are caught.
• Live at 805c818902, iOS 27.1 simulator: build:xcuitest:ios wrote the manifest with only plutil execs, and its key matched xcodebuild -version and xcrun --show-sdk-*. open and snapshot -i succeeded. The runner ran test-without-building with no rebuild.
• Independent review findings applied.

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